Therapeutic lesions in living bodies have been accomplished for many decades using radio-frequency (RF) and other forms of energy. The procedures have been particularly useful in the field of neurosurgery, typically where RF ablation electrodes (usually of elongated cylindrical geometry) are inserted into a living body. A typical form of such ablation electrodes incorporates an insulated sheath from which an exposed (uninsulated) tip extends.
Generally, the ablation electrode is coupled between a grounded RF power source (outside the body) and a reference ground or indifferent electrode for contacting a large surface of the body. When an RF voltage is provided between the reference electrode and the inserted ablation electrode, RF current flows from the ablation electrode through the body. Typically, the current density is very high near the tip of the ablation electrode, which heats and destroys the adjacent tissue.
Ablation electrode techniques, including the theory behind the techniques and many applications of the techniques are described in various papers, specifically see, (1) Cosman et al, xe2x80x9cTheoretical Aspects of Radiofrequency Lesions in the Dorsal Root Entry Zonexe2x80x9d Neurosurg 15:945-950, 1984 and (2) Cosman E. R. and Cosman B. J.: xe2x80x9cMethods of Making Nervous System Lesions, in Wilkins R H, Rengachary S S (EDS): Neurosurgery, New York, McGraw-Hill, Vol. III, pp. 2490-2498, 1984.
In the past, RF ablation electrodes have incorporated temperature sensors, for example, in the form of a thermistor or thermocouple. In that regard, see U.S. Pat. No. 4,411,266 (1983, Eric R. Cosman). Typically, the sensor is connected to a monitoring apparatus for indicating temperature to assist in accomplishing a desired lesion. As generally known, for a given tip geometry and tip temperature, lesions of a prescribed size can be made quite consistently. In that regard also, see U.S. Pat. No. 4,411,266, (1983, Eric R. Cosman).
Over the years, a wide variety of RF electrode shapes and configurations have been used, for example, several current forms are available from Radionics, Inc., located in Burlington, Mass. Such electrodes have been used to accomplish lesions in a wide variety of targets within the body, including the brain, the spinal column and the heart.
However, a limitation of prior electrode ablation systems relates to the temperature of the tip. Specifically, prior ablation electrodes of a given tip geometry never should effectively exceed a temperature of 100xc2x0 C. At that temperature, the surrounding tissue will boil and char. Also, uncontrolled disruption, such as hemorrhage and explosive gas formation, may cause extremely hazardous and clinically dangerous effects on the patient. Consequently, the lesion size for a given electrode geometry generally has been considered to be somewhat limited by the fact that the tissue near the tip must not exceed 100xc2x0 C.
Essentially, during RF ablation, the electrode temperature is highest near the tip, because the current density is the highest at that location. Accordingly, temperature falls off as a function of distance from the electrode tip, and except for possible abnormalities in tissue conductivity and so on, in a somewhat predictable and even calculable pattern. As an attendant consequence, the size of RF lesions for a given electrode geometry have been somewhat limited.
One proposed solution to the limitation of lesion""s size has been to employ xe2x80x9coff-axisxe2x80x9d electrodes, for example the so called Zervas Hypophysectomy Electrode or the Gildenberg Side-Outlet electrode, as manufactured by Radionics, Inc., Burlington, Mass. However, such systems in requiring multiple tissue punctures, increase the risk of hemorrhage, severely the prolong the time of surgery and increase the level of delicacy. Also, an umbrella of off-axis lesions may not produce a desired homogenous or uniform lesion. Accordingly, a need exists for an ablation electrode system capable of accomplishing enlarged lesions (radius and volume).
Considering lesion size, the papers of Cosman et al. (cited above) describe producing lesions in the brain of up to 10 to 12 millimeters by using very large electrodes. Yet, a need exists to attain much larger lesions. For example, in the liver, cancerous tumors may exceed 20 or 30 millimeters and may be clearly visible, as by tomographic scanning. Accordingly, a need exists for the capability to heat such tumors destructively with a minimum number of electrode insertions and heating episodes.
In general, the system of the present invention is directed to an improved system for accomplishing ablations in the body. The system offers a capability for controlled and modified temperature distribution as a function of distance from the ablation electrode so as to xe2x80x9cthrow outxe2x80x9d or extend the heat to much larger distances while generally preserving the safety and control of the lesion process. The system enables controlling the temperature at a heating terminal, as for example the tip of an ablation electrode. For example, in disclosed embodiments, the temperature of the electrode tip (heat device) is controlled by incorporating a mechanism to cool the tip so as to reduce the excessive temperatures of the ablation process adjacent to the tip. For example, by the incorporation of a controllable, externally modulated agent (fluid) for secondary cooling of the tip, control is accomplished and in that regard, excessive heating of tissue near or adjacent the tip is reduced. Specifically, disclosed embodiments incorporate a cooling component which enables cooling of the ablation electrode and the tissue just adjacent to the electrode so as to modify the thermo distribution of heat disposition in the tissue and attain larger lesions. Essentially, the ablation energy dissipated in the tissue as heat can be effectively increased as a result of cooling at the working surface. As a result, the ablation volume is increased. Forms of cooled-tip, high frequency, electrodes as disclosed herein are well suited for percutaneous minimal invasive ablation of tumors. Specific embodiments are disclosed to be useful in thermo surgical settings which possess physical characteristics affording improved control and handling. Particular assemblies of cannula, fluid handling structures, irrigating and perfusion devices, radiofrequency cannula, and thermo probes are disclosed which afford the possibility of constructing various practical thermo-surgical applicators capable of effective operation. Additionally, as disclosed herein, control may be enhanced by the utilization of a computer, as with graphics and display capability to control, monitor or feedback parameters of the thermo surgery, also to preplan the ablation, or map, fuse or update images from one or more image scanners before, during or after the ablation process.